1. Field of the Invention
The present invention relates to a diesel particulate oxidizer disposed in an exhaust system of a diesel engine and a control system for its regeneration.
2. Description of the Prior Art
In the exhaust gas of a diesel engine are contained particulates which are a flammable carbide compound of fine particles and which are the main cause of converting the exhaust gas into a black smoke.
The particulates referred to herein indicate flammable fine particles consisting principally of carbon and hydrocarbons, having an average diameter of about 0.3 .mu.m and undergoing self-ignition and burning (hereinafter referred to as "self-burning") at a temperature not lower than about 500.degree. C. (not lower than 350.degree. C. in the presence of an oxidation catalyst).
The particulates undergo self-burning at high speed and high load of a vehicle at which the exhaust gas temperature rises above 500.degree.. But, during normal operation or idling (which occupies more than 90% of vehicular operation) in which the exhaust gas temperature does not reach 500.degree. C., the particulates are discharged to the atmosphere directly.
However, since the particulates may be harmful to the human body, various studies have recently been made about mounting a diesel particulate oxidizer for collecting the particulates in the exhaust gas in the exhaust passage of a vehicular diesel engine.
As a trap carrier of a diesel particulate oxidizer there is used a trap carrier provided with a depth collection type heat-resistant ceramic foam with catalyst containing platinum, palladium or rhodium (which foam comprises two plates having an oval, elliptic or rectangular section). This diesel particulate collecting member will be hereinafter referred to simply as "DPO".
By using the DPO, the particulates are collected and deposited, tending to block the exhaust passage. For regeneration of the DPO to avoid such inconvenience, various studies have been made about the mechanism of accelerating the recombustion of the particulates.
As an example of such regeneration assisting mechanism, the fuel injection timing is retarded, an intake throttling is made, or the quantity of the exhaust gas recirculated is increased. But, it is desirable to inhibit the acceleration of regeneration of the DPO according to the status of the engine.
More particularly, if regeneration of the DPO is performed at a high engine load (in the vicinity of full open of the accelerator), the accelerating performance of the engine (that of the vehicle carrying the engine thereon) is deteriorated. And if regeneration of the DPO is conducted in a low revolution region of an engine, the drivability of the vehicle which carries the engine thereon cannot be ensured. Moreover, even if the regeneration assisting mechanism is retarded in a low speed and low load region, the particulates cannot undergo self-burning, thus resulting in unnecessary control being made.
Further, if promotion of the DPO regeneration is made in a cold state of an engine, the exhaust gas temperature does not fully rise, thus resulting in the formation of a blue smoke or the regeneration efficiency becoming very poor.
In a conventional DPO regeneration system, for starting or stopping the operation of a regeneration assisting mechanism, the use of a pressure sensor may be effective which detects an exhaust gas pressure in an exhaust passage upstream of the DPO.
In such diesel engine exhaust pressure detecting means, however, the steam, soot and SOx contained in exhaust gas are deposited in an exhaust pressure detecting pipe or in a water trap disposed therein, thus resulting in water freezing at a cold place. Further, the detection pipe may be clogged with soot, thus causing deterioration of the pressure sensor performance; for example, it becomes impossible to effect the exhaust pressure detection, impossible to make an exact detection, and a delay of transfer occurs. Reduction of durability also results.
As the foregoing regeneration means, it has been proposed to retard the fuel injection timing, make intake throttling and increase the quantity of exhaust gas recirculated. Even with such proposal, since there is an engine operation region in which the exhaust gas temperature does not fully rise, the particulates collected by the DPO do not burn long, causing an overloading condition and decrease of the output by clogging of the DPO.
If the particulates deposited in a large volume are burnt forcibly, the DPO will melt and damage because of combustion of the particulates in a large quantity.
In conventional DPO regeneration systems, moreover, if it is tried to regenerate DPO in a certain specific operation condition (e.g. several days' idling), the temperature of the exhaust gas from an engine will not rise sufficiently and so the particulates adhered to the DPO are not burnt. Consequently, the DPO may be clogged.
In conventional DPO regeneration systems, therefore, it is considered to use a diesel particulate burning device which brings engine into a high idling condition (e.g. 3,000 rpm) forcibly during stop of a vehicle. However, if such high idling condition is created, the engine noise will become louder, and so the driver may feel uneasy. Moreover, a retard would cause a temporary production of aldehyde, etc. and offensive odor in the exhaust gas. Thus, there are problems in point of commercial value.